1. Field of the Invention
The present invention relates to a hologram, comprising a hologram film in which a diffraction grating has been recorded, that can be used as a screen, an optical reflection element or the like, and to a hologram production process for continuous production of holograms which are copies of the same photographed object original.
2. Description of the Related Art
Hologram films made of photopolymers and gelatin dichromate are well known in the prior art as hologram optical elements (HOES) employing volume phase-type holograms.
When such hologram films are used for various hologram optical elements, increased nick resistance, moisture resistance and solvent resistance of the hologram films are often provided by joining the hologram film to a protective material such as a substrate made of glass or a polymer film via a bonding material, leaving the hologram film in a sealed state.
In cases where the hologram film is made of gelatin dichromate, it has been proposed to use epoxy-based adhesives with excellent moisture resistance as the bonding material. For hologram films made of photopolymers, bonding materials proposed for use have included an adhesive containing no plasticizer to prevent migration of the solvent from the polymer film, etc. (Japanese Unexamined Patent Publication (Kokai) No. 6-56484) and an adhesive comprising a polyfunctional acrylate and a polymerization initiator (Japanese Unexamined Patent Publication (Kokai) No. 3-157684).
The following are well-known examples of applications wherein various hologram optical elements are used as display apparatuses.
As a first example there may be mentioned a hologram screen wherein a hologram film bearing a recorded light diffuser is used as a screen.
As shown in the attached FIG. 2, the hologram film can be fabricated by irradiating a photopolymer, gelatin dichromate or the like with an object beam composed of scattered light created by passing light through a light diffuser, and a reference beam consisting of non-scattered light, and recording an interference pattern functioning as a diffraction grating, which is produced by the object beam and the scattered light on the photopolymer, gelatin dichromate, etc.
The hologram film is reinforced with a protective material or bonding material as described above to construct a hologram screen. In the hologram screen, irradiation of an irradiated beam containing image data onto the screen can display an image because of diffraction and scattering of the irradiated beam on the screen.
As a second example there may be mentioned a reflection-type hologram reflector element which employs a hologram film on which a recorded plane mirror, concave mirror, convex mirror, etc. has been recorded, and which functions as a mirror.
This hologram film can be fabricated by irradiating onto a photosensitive material an object beam obtained by reflection at a concave mirror or convex mirror and a reference beam consisting of non-reflected light.
This type of hologram film is reinforced by a protective material or bonding material as described above to construct a hologram screen or hologram reflector element.
In this hologram reflector element, reflection of an irradiated beam containing image data can display a virtual image behind the hologram reflector element. A lens function may also be provided to allow magnification or reduction of the virtual image depending on the type of concave mirror or convex mirror.
The holograms described above, however, are prone to heat shrinkage of the polymer films when the use environment is under a high temperature. In such cases, since the substrate of glass, etc. undergoes virtually no heat shrinkage, stress is created between the substrate of glass, etc. and the polymer film, and this stress causes a deformation of the hologram film that can alter the gradient of the interference pattern in the hologram film.
Interference patterns with various gradients are recorded in hologram films, and light irradiated onto the holograms is diffracted because these interference patterns function as diffraction gratings. The interference patterns become inclined in response to any deformation of the hologram films, and their directions are thus altered.
When light is irradiated onto a hologram film, the light is diffracted due to the interference pattern; however, even if it is diffracted by the same interference pattern before and after deformation of the hologram film, it is diffracted in a different direction after deformation than before deformation because the gradient of the interference pattern is altered. Consequently, as shown in the attached FIG. 7, a phenomenon occurs whereby the peak of the hologram spectral characteristics is shifted toward the long wavelength end or short wavelength end after heat shrinkage occurs.
A screen or reflector element made of the hologram described above utilizes a diffraction effect due to the interference pattern in the hologram film, and therefore when deformation of the hologram film occurs due to heat shrinkage of the polymer film, a problem occurs in that a hue difference is produced between the irradiated beam which is irradiated and the image obtained by diffraction or reflection, or the hue is different between the outer perimeter and inside of the hologram, making the image appear deformed.
Naturally, this problem does not occur if the protective material encapsulating the hologram film is composed entirely of the same material or of a substance with the same degree of heat shrinkage, but this is not practical either because of the following problems.
One of these problems is that when the hologram is a film, the hologram twists, altering the hue of the image and producing deformation of the image. It is therefore necessary to support the hologram film with a substrate. The production process becomes more complicated with a structure wherein the hologram film is sandwiched between substrates.
As a practical operating method, when the hologram screen is placed onto window glass or the like, the semi-completed product comprising the hologram film, polymer film and bonding material is affixed onto the window glass. It is not practically possible to affix the hologram film bonded to the substrate onto another window glass or the like without including air bubbles, etc. When air bubbles have been included, the air bubbles are seen over the image, making it difficult to view the image.
Moreover, when the hologram is used as a reflector element and a plane mirror has been recorded on the hologram film, a lens effect (concave mirror/convex mirror effect) is produced which magnifies or reduces the reflected virtual image of the hologram, or when a concave mirror or convex mirror has been recorded it has often been impossible to obtain the designed lens effect (see Embodiment 3 below).
These problems occur most notably around the perimeter of the hologram where stress readily accumulates (see Embodiments 2 and 3 below).
In light of these problems of the prior art, it is an object of the present invention to provide a hologram with excellent heat resistance.
The present invention also relates to a hologram production process for continuous production of holograms which are copies of the same photographed object original.
A conventional process for continuous production of the same hologram is known whereby a photographed object original is continuously copied onto a film-like, long photopolymer (Japanese Unexamined Patent Publication (Kokai) No. 9-90857).
As shown in FIG. 13 explained below, after the film-like photopolymer is fed to a supply roller and the photopolymer is affixed to the photographed object original, it is exposed to laser light to copy the photographed object original onto the photopolymer. This converts the photopolymer to a hologram.
The hologram is then released from the photographed object original and affixed onto a protective film to protect the soft, easily damaged hologram. It is then wound up on a winding roller and the hologram is stored in a wound state.
This allows continuous production of holograms which are copies of the same photographed object original. The holograms obtained by this process are also long, and are therefore used after appropriate cutting.
By the conventional process described above, however, there is a risk of uneven attachment of the protective film onto the hologram due to slight vibrations, etc. of the protective film, and this has resulted in possible string-like appearance defects in the hologram. Emboss-like appearance defects have also been a risk, because irregularities in the attachment roller are transferred to the soft hologram.
As a more detailed explanation, while an appropriate pressing force is applied when affixing the protective film to the hologram, the pressing force sometimes exceeds the appropriate pressing force because of variation due to vibrations when the protective film moves, irregularities in the roller, etc.
In such cases, deformations 351, 301 are produced in the protective film 215 and hologram 201, as shown in FIG. 16B explained below. Since the hologram 201 is soft the deformations 301 sometimes disappear by the elasticity of the hologram 201 itself, but in most cases deformations are almost never eliminated from the hologram 201 once it has been deformed, because the protective film 215 acts to secure the deformations in the hologram 201.
When a hologram screen is constructed with a hologram in which such deformation has occurred, and an image beam 320 is irradiated onto the hologram screen by a projector 232 as shown in FIG. 14 explained below, there has been a risk of string-like blemishes 291, emboss-like blemishes 292 and the like being reflected on the hologram screen 209 as shown in FIG. 15B, which can be clearly seen by the viewer 238. These blemishes 291, 292 overlap the image 310, and thus risk noticeably impairing the appearance of the image 310.
In light of these problems of the prior art, it is another object of the invention to provide a process for producing holograms which can prevent occurrence of appearance defects.